SCN5A encodes the alpha subunit of the human voltage-dependent Na channel (hNaV1.5) found in heart. We have made the novel observation that up to four very common variants of NaV1.5 exist in human heart and at least some have functional implications. Mutations in this channel also cause sudden cardiac death in the congenitally acquired long QT syndrome (LQT3) and the Brugada Syndrome (BS). We have recently characterized four novel SCN5A mutations and found: 1) Two in Sudden Infant Death Syndrome (A997S, R1826H) are LQT3.2) LQT3 (M1766L) and BS (G1743) mutations have expression defects "rescued" by antiarrhythmic drugs. 3) M1766L has normal or absent current depending on the variant NaV1.5 background used to test it. We propose to investigate further the extent and mechanisms of expression defects and their "rescue" and the importance of "background". Through collaboration with Dr. Ackerman at Mayo Clinic we also have >20 additional novel SCN5A mutations to investigate for novel functional defects and arrhythrnia mechanism. We will make and express these channels in cell culture, define function by voltage clamp and immunocytochemistry, and correlate molecular function with clinical phenotype through arrhythmia mechanism. In Aim 1 we will investigate the expression and function of wild type variants, and also how mutant channel expression and function depends upon the background clone. In Aim 2 we will study novel mutants. In Aim 3 we will investigate mutants with "gain of function" and test the hypothesis that late current decay in LQT3 correlates with enhanced rate dependent QT interval adaptation, later onset, and better prognosis than mutations without late current decay. In Aim 4 we will investigate mutations with "loss of function", the mechanism for loss of function including novel trafficking defects, and test the hypothesis that this loss can be "rescued" by drugs. In Aim 5 we will co-express mutations with the beta1 and beta3 subunit, and assess effects of PKA stimulation, to test the hypothesis that these areas are critical to the mechanism of action. These studies on the novel findings will have implications for arrhythmia mechanism and genotype-phenotype correlation in both mutation arrhythmia syndromes and more generally for the variants in "normal' hearts that may generate insight into genetic predisposition to acquired arrhythmia. At a more basic level these "natural" experiments will contribute to understanding the structure-function relationship of this important channel.